1. FIELD OF THE INVENTION
The present invention relates to facilitating the maintenance of nuclear power systems and particularly to minimizing the exposure to radioactivity of service personnel working in the vicinity of the steam generator of a nuclear power system during the performance of routine periodic maintenance thereon. More specifically, this invention is directed to radiation shielding "door" assemblies which may be rapidly and easily installed over access openings which may be rapidly and easily installed over access openings which are provided in steam generators after removal of the covers such access openings pursuant to a maintenance and/or inspection procedure. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. DESCRIPTION OF THE PRIOR ART
While not limited thereto in its utility, the present invention has been designed for use in, and has special utility for, the servicing of the steam generators of nuclear power systems, particularly such systems which employ pressurized water type reactors. Such steam generators comprise a pressure vessel having a lower plenum area which is divided into two halves. A coolant, which has been heated in the reactor, is delivered to one of the plenum halves and is then circulated, via a bundle of tubes, through the steam generator vessel. The coolant is subsequently discharged from the other plenum half and returned to the reactor. During passage through the steam generator tube bundle, heat from the reactor coolant is transferred to water under pressure which subsequently flashes to steam for driving a turbine. Nuclear Regulatory Commission rules require periodic inspection of the steam generators of nuclear power systems. To this end, the steam generator vessel is provided with access openings, known in the art as "manways", in the lower plenum area. During normal operation, these access openings are sealed by means of covers. In order to perform inspections, after the vessel has cooled and the "primary loop" has been drained of reactor coolant, the existing manway covers are removed.
Once access to the interior of the steam generator pressure vessel is possible, a number of different procedures may have to be performed within the vessel. However, since the interior of the steam generator vessel is classified as a highly radioactive environment, maintenance personnel may work in the lower plenum area for only short periods of time. Thus, maintenance personnel will enter the lower planum area primarily for the purpose of erecting remotely controllable equipment which has been designed for rapid installation. This equipment will thereafter be operated from the exterior of the steam generator with power and commands being delivered to the remotely controllable equipment via flexible cables and conduits. The types of procedures which may have to be performed from within the lower plenum area of a steam generator vessel include non-destructive testing, steam generator tube pulling, steam generator tube plugging, installation of sleeves in steam generator tubes and the installation of nozzle dams to prevent backflow of coolant from the reactor pressure vessel should it be necessary to flood the reactor in order to perform separate operations thereon. The non-destructive testing will typically comprise ultrasonic and/or eddy current examination of the interior of a preselected percentage of the steam generator tubes. For example, 3% of the tubes will be tested to determine if there is any reduction in effective wall thickness, i.e., cracks, pits, or corrosion, of greater than 20% or any growth in effective wall thickness, i.e., scaling or other deposits, of more than 20%. If a preselected number of the tested tubes are found to exceed the set limits for increased or reduced wall thickness, an additional percent of the tubes will be tested.
Radiation will "stream" outwardly from an opened manway. Accordingly, steps must be taken to minimize the exposure to the manway "shine" of service personnel who are working on the operating platform in the vicinity of the manway. In this regard, it is to be noted that the manway covers cannot simply be re-installed to afford such protection since it is necessary for the above-mentioned cables and conduits to pass from the exterior control and monitoring apparatus to the interior of the steam generator. Additionally, to prevent condensation from forming therein, it is mandatory that the interior of the steam generator be constantly ventilated during the time it is out-of-service. Prior attempts to provide the requisite radiation shielding in the vicinity of an open manway have largely been limited to establishing a restricted area on the work platform outside of the steam generator and hanging lead blankets over the open manway to reduce the "shine" and "streaming" when access to the manway was not necessary. This has not proven to be a satisfactory procedure since the blankets take a long time to install, and the installers are subjected to radiation during the installation, and because the work platform is not large enough to store the blankets along with inspection/maintenance equipment when the blankets are taken down to permit access to the steam generator.
A European nuclear power system operator has employed a pair of rectangularly shaped hinged radiation door sets which are attached to specially designed rectangular mounting brackets. These mounting brackets are designed for attachment to the flange which extends around the manway and defines the manway opening, a pair of oppositely disposed mounting studs being required for the installation of each mounting bracket. These mounting brackets could be installed in only one orientation. A significant problem with such prior art radiation door sets is precipitated by the fact that the studs by which the manway cover is affixed to the flange which extends about the manway opening are often corroded to the point that they either are stuck and cannot be removed or break during the cover removal operation. In either case, there is a high degree of probability that the mounting brackets for prior art radiation shielding doors could not be installed without performing work in the vicinity of the open manway in order to provide the two-pair of oppositely disposed studs which are required for installation of the mounting brackets. The performance of such work in the vicinity of an open manway, as in the case of installation of hanging lead blankets, would expose the service personnel to maximum allowable radiation exposure within a very short time.